This article examines grammar instruction produced on the fly by a teacher in response to students' questions in a Dutch as foreign language classroom. Such sequences merit attention because they present teachers with the opportunity and the challenge to provide unplanned instruction on an aspect of grammar to which a student has shown herself to be attending. Using the tools of conversation analysis, we examine two sequences in which a student initiates talk about Dutch grammar and the teacher constructs a mini-lesson using talk, gesture and writing on the blackboard. In first, the teacher produces a paradigm, a practice used widely in linguistics and L2 education. In the second, he produces a contrastive pair, a common practice in linguistics. We consider tensions entailed in on-the-fly grammar instruction produced in response to students' questions.